An optical transmitter that is used in an optical communication system includes, for example, a light source, an optical modulator, and driver circuit for the optical modulator. A laser diode, other semiconductor laser, or the like is used as the light source. The optical modulator modulates continuous wave (CW) light output from the light source. For example, an electro absorption modulator (EAM), a Mach-Zehnder modulator (MZM), or the like is used as the optical modulator. The driver circuit for the optical modulator supplies a driving signal to the optical modulator.
In an optical transmitter that is used for, for example, high-speed data communication of 28 Gbps or 40 Gbps, a high-speed signal transition with a falling time and a rising time of 10 ps or less is required in an eye pattern of an optical output signal output from the optical transmitter. Further, for an input and an output of a driver circuit for an optical modulator, a small reflection coefficient (S parameters; S11 and S22) up to a frequency of about 20 GHz is required. Therefore, a traveling wave amplifier (TWA) is used in the driver circuit for an optical modulator. The traveling wave amplifier is capable of high-speed operation and is suitable for obtaining a small reflection coefficient up to a high frequency range. In order to improve high frequency characteristics of the traveling wave amplifier, it is necessary for a length of an internal transmission line to be lengthened, but providing a long transmission line is not preferable for miniaturizing the optical transmitter.